Perpend stone

Perpend stone (parpen, parpend, perpin, and other spellings),bond stone, or tie stone is a structural element building term used by stonemasons and brick masons. Perpend is also a piece in brickwork also called a cross joint or when extending through the entire wall a transverse joint or perpend bond.

Usually stone walls are built with two layers of stone, an inner and an outer layer, with the space between them sometimes filled with rubble. A perpend stone is a longer stone that extended through the entire wall's width, from the outer wall to inner wall, which serves to lock the two wall layers structurally together.